1. Field of the Invention
The present invention relates to a circularly polarized wave microstrip antenna having a dielectric substrate with a ground conductor on one surface and a radiation conductor on the other surface, and to a frequency adjustment method therefor.
2. Description of the Prior Art
Conventionally, a circularly polarized wave microstrip antenna is known in which a projection or a notch for generating a circularly polarized wave is formed at a specified position on the periphery of a radiation conductor for feeding electric power to a power feeding point eccentrically located on the radiation conductor, as disclosed in the Japanese Patent Laid-open Publication (unexamined) 3-80603.
FIG. 12 shows such a conventional circularly polarized wave microstrip antenna.
In the conventional circularly polarized wave microstrip antenna 7 shown in FIG. 12, a ground conductor (not shown) is provided on the entire part of one surface of a circular dielectric substrate 4, and a radiation conductor 8 is provided at a center position on the other surface of the substrate 4. With the above construction, an electric power is fed from the ground conductor to a feeding point P located on the radiation conductor 8 by way of a coaxial cable (not shown), wherein the feeding point P is provided radially eccentrically to the center point O.
The radiation conductor 8 is circular in form and is provided with rectangular projections 8a through 8d for radiating a circularly polarized wave at four peripheral portions where the radiation conductor 8 intersects two straight lines m and n, which are at an angle of .+-.45.degree. with respect to a straight line M passing through the center point O and the feeding point P.
It is conventionally known that, when the above-mentioned projections 8a through 8d are reduced in length, the axial ratio between the major axis and the minor axis of the circularly polarized wave microstrip antenna varies and the resonance frequency at which the axial ratio is minimum is made higher. By taking advantage of the above-mentioned characteristics, adjustment of the axial ratio and the resonance frequency of the circularly polarized wave microstrip antenna 7 can be effected.
In more detail, the resonance frequency of the circularly polarized wave microstrip antenna 7 is generally determined by the diameter R of the radiation conductor 8, the dielectric constant .epsilon. of the dielectric substrate 4, and the thickness t of the dielectric substrate 4. Therefore, by setting the above-mentioned three parameters so that the initial frequency (unadjusted resonance frequency) of the circularly polarized wave microstrip antenna 7 is made slightly lower than a desired frequency, and by abrading the aforesaid four projections 8a through 8d by the same amount so as to reduce the length Lt of each projection, the axial ratio is adjusted to a minimum and the resonance frequency at which the axial ratio is minimum is made gradually higher so as to achieve the intended resonance frequency.
Although the above-mentioned conventional circularly polarized wave microstrip antenna 7 may be used for adjusting the resonance frequency to the desired frequency by gradually raising the resonance frequency through abrading the projections 8a through 8d for generating a circularly polarized wave, since there is no adjustment member for lowering the resonance frequency, it is very difficult to adjust the resonance frequency by gradually lowering the resonance frequency. Therefore, when the projections 8a through 8d are excessively abraded thereby making the resonance frequency exceed the desired frequency, the frequency of the antenna cannot be further adjusted thereby reducing the yield in the manufacturing process.
Furthermore, since the axial ratio and the resonance frequency of the circularly polarized wave microstrip antenna are adjusted at the same time by abrading the aforesaid projections 8a through 8d, it is difficult to achieve a balanced adjustment between both these factors.
FIG. 13 shows another conventional circularly polarized wave microstrip antenna, which is similar to that of FIG. 12, and, therefore, similar parts of FIG. 13 are designated by the same reference numerals as those of FIG. 12.
In FIG. 13, a rectangular dielectric substrate 9 is used instead of using a circular one. The radiation conductor 8 is circular in form having a radius R and is provided with rectangular projections 81a and 81b on the periphery of the radiation conductor on a line M2 inclined at an angle of 45.degree. with respect to a straight inclined at an angle of 45.degree. with respect to a straight line M1 passing through the center point O and the power feeding point P, and notches 82a and 82b formed on the periphery of the radiation conductor 8 on a line M3 inclined at an angle of -45.degree. with respect to the straight line M1.
The projections 81a and 81b as well as the notches 82a and 82b serve as mode degeneration separation elements for generating a circularly polarized wave, and by changing the length of each of the projections 81a and 81b and the depth of the notches 82a and 82b, the axial ratio between the major axis and the minor axis of the circularly polarized wave microstrip antenna is varied, also varying the resonance frequency at which the axial ratio is minimum.
In more detail, when the length L1 of each of the projections 81a and 81b is reduced, the resonance frequency is made higher, or when the depth L2 of each of the notches 82a and 82b is increased, the resonance frequency is made lower.
In view of the above fact, there has been conventionally proposed a method of adjusting the resonance frequency of the circularly polarized wave microstrip antenna by adjusting the length L1 of the projections 81a and 81b and the depth L2 of the notches 82a and 82b through abrading the projections 81a and 81b and the notches 82a and 82b.
In the above-mentioned conventional circularly polarized wave microstrip antenna 7, both the axial ratio and the resonance frequency of the circularly polarized wave are adjusted at the same time by abrading the projections 81a and 81b and the notches 82a and 82b for generating a circularly polarized wave, and, therefore, it is difficult to adjust both the above-mentioned factors keeping a balance between the two.
When the length L1 of each of the projections 81a and 81b and the length L2 of each of the notches 82a and 82b are changed, an influence is exerted, for example, on such characteristics as the input impedance and the directivity of the antenna, and, therefore, it is difficult to adjust only the frequency.